The present invention relates to a press and a method for modifying a press for use in the press section of a papermaking, cellulose or board manufacturing machine. More specifically, the invention relates to the field of shoe presses, being a type of presses having a press nip extended in the machine direction.
A shoe press generally comprises a rotary press roll, a substantially stationary pressure shoe, and a press belt running in an endless path around the pressure shoe between this and the press roll and having a substantially impermeable inner surface facing the pressure shoe. The pressure surface of the shoe facing the press roll is arcuate by a curvature corresponding to the radius of the press roll. The press nip formed between the press roll and the shoe is intended to receive, between the press roll and the outer side of the press belt, a web, such as a paper web, from which water is to be removed. The water which is pressed out of the paper web by the press is initially taken up by one or more press felts, generally one press felt in the form of a sandwich structure provided on each side of the paper web. In operation, the rotating roll feeds the paper web, the felt or felts, as well as the press belt jointly through the press nip.
The extent of the pressure surface of the shoe in the machine direction may be in the order of 25 cm (10 inches), which is several times longer than the nip in a traditional roll press with two rolls, and (for a given web speed) results in a corresponding increase of the press time. Hence, a shoe press enables considerably enhanced dewatering of the paper web.
In a shoe press, between the shoe and the inner surface of the press belt, there is traditionally provided for friction-reducing oil lubrication by means of an oil film. The oil film is produced by conducting pressurised oil through one or more oil ducts provided in the interior of the shoe and opening at its pressure surface. The press belt serves to maintain the oil film established between the press belt and the shoe in place and, therefore, must be oil-impermeable. In this way, the oil is also prevented from fouling the paper web and the felt or felts.
Another demand placed on the press belt in a shoe press is that the inner surface of the press belt must be smooth to expose a good sliding surface to the shoe. As known in the art, a good sliding surface can be achieved by providing a base weave of the press belt with a layer of plastic, which then also makes the press belt oilproof. Such press belts provided with a plastic layer are known from EP-A-01,194,601 (Albany Int. Corp.), U.S. Pat. No. 4,564,551 (Best), U.S. Pat. No. 4,946,731 (Dutt), U.S. Pat. No. 4,559,258 (Kiuchi), and JP 63-247061 (Ichikawa Umou Co. Ltd).
In addition to the above-mentioned two demands placed on the press belt in a shoe press--oil tightness and a smooth inner sliding surface--the belt must also be strong to have a long service life and be dimensionally stable so as not to be stretched during mounting or in operation. Moreover, the press belt must be given a uniform thickness when manufactured.
To sum up, the following demands are thus placed on a press belt in a shoe press:
1. Oil tightness. PA1 2. Smooth inner sliding surface. PA1 3. Long life. PA1 4. Dimensional stability. PA1 5. Uniform thickness.
Hitherto, it has been difficult to meet all these demands (1-5) on a press belt in a shoe press. Especially, a dimensionally unstable press belt causes problems in shoe presses of the short-belt type, which in the context of this invention means a shoe press which, in addition to the features mentioned above, is distinguished by the press shoe being integrated in the outer periphery of a usually cylindrical, non-rotating element, the outer periphery of which defines the endless path of the press belt. Rotary sealing means are arranged at each end of the cylindrical element in order, together with the running press belt, to retain the oil film in a closed space. As compared with shoe presses of the long-belt type, which lack the above-mentioned non-rotating, cylindrical elements and the rotary sealing means and in which the press belt instead runs about an assembly of separate guide rollers, a short-belt type shoe press is advantageous in that the oil which the press belt draws off from the oil film in the press nip will be retained inside a closed system. In a long-belt type shoe press, special measures must be taken to remove such entrained oil from the press belt and also to collect the removed oil, which makes a shoe press of the long-belt type more complex and expensive.
The reason why dimensional instability of the press belt in particular entails problems in a shoe press of the short-belt type will now be explained.
If the press belt is stretched in the running direction, i.e. circumferentially about said cylindrical element, this may result in an impermissible increase of the diameter of the endless press belt with consequent operational disturbance. Further, manufacturing a press belt of a length exactly corresponding to the diameter of the cylindrical element obviously poses problems. As to the length of the press belt, which traditionally lies within a certain tolerance range, there are two contradictory desiderata. It is desirable, on the one hand, that the press belt is easy to mount and, on the other hand, that when mounted it has no radial play with respect to the cylindrical element about which the press belt runs. The first desideratum is satisfied by means of a press belt which is in the upper part of the tolerance range, whereas the second desideratum is satisfied by means of a press belt which is in the lower part of the tolerance range.
In a shoe press of the short-belt type, the press belt is also stretched transversally of its running direction on the cylindrical element, i.e. in the axial direction thereof, and in addition to the above-mentioned problem of stretching in the circumferential direction, the press belt, also, must not be stretched too much in the axial direction, since axial stretching gives rise to practical problems in the axial tensioning of the press belt when being mounted.
A problem common to shoe presses of both the short-belt type and the long-belt type relates to difficulties in taking care, in the press nip, of the water removed from the paper web. More specifically, it is difficult to provide an open, incompressible volume sufficient for receiving the water from the press felt or felts.
It is known to provide such an incompressible, open water-receiving volume directly in the outer surface of the press belt, i.e. such that the press belt performs the double functions of sealing against the oil film and of taking care of water pressed out of the web to remove it. U.S. Pat. Nos. 4,946,731 and 4,559,258 mentioned above describe press belt structures having a base weave completely enclosed by an impermeable layer of plastic, whose outer surface is formed with substantially incompressible water-receiving grooves. The above-mentioned U.S. Pat. No. 4,564,551 and JP 63-247061 describe press belt structures having a base weave whose inner side is provided with an impermeable layer of plastic and whose outer side has a water-receiving structured surface formed by the base weave itself.
JP 63-247061 also describes the use of a separate dewatering belt in the form of a wire cloth which, in the press nip, runs between the structured outer surface of the press belt and a press felt and which, outside the press nip, runs about separate guide rollers.